This invention relates to apparatus and method for determining the rate of flow of a fluid by measuring the electrical potential difference developed in the fluid as the fluid moves through a magnetic field and for causing an electrically conductive fluid to flow by application of an electric potential and a magnetic field.
When a fluid that is at least weakly electrically conductive flows in a magnetic field oriented transversely to the flow direction, an electric potential is generated in a fluid along an axis orthogonal to both the direction of flow and the magnetic field. Conversely, application an of orthogonally oriented electric potential and a magnetic field to a conductive fluid generate forces in the fluid that tend to make it move in a direction orthogonal to both the potential and the field. These interactions can be exploited in making a variety of transducers that comprise both magnetic flow meters and electro-magnetic pumps.
Prior art in-line magnetic flow meters are well known instruments in which two electrodes contacting the fluid are spaced apart transverse to a direction of fluid flow in a pipe or tube (i.e., the axis of the pipe) and magnets are arranged to provide a magnetic field orthogonal to both the direction of fluid flow and the line along which the electrodes are spaced. An electrical potential difference developed responsive to fluid flow in a magnetic field is sensed at the electrodes, processed in suitable signal processing electronic circuitry, and provided as a measure of flow rate. The potential difference at the electrodes increases with the path length between electrodes and increases with the magnetic field. In conventional magnetic flow meters the electrode spacing is generally selected to be approximately the same as the diameter of the pipe and the magnetic field is commonly provided by two coils of wire located on opposite sides of and outside of the passage. Sophisticated electronics are used to energize the magnetic field, amplify the tiny flow-related signals generated, and reject various noise and drift signal components which would otherwise degrade measurement precision. These meters are characterized by an unobstructed flow passage offering very low pressure drop and high tolerance to solids in the fluid, high measurement precision, high power consumption, and high cost.
Prior art in-line electro-magnetic pumps are often used for pumping electrically conductive fluids (e.g., molten metal). Apparatus of this sort generally comprises a pair of electrodes spaced out along a line orthogonal to a desired direction of flow and magnets for producing a magnetic field orthogonal to both the direction of fluid flow and the line along which the electrodes are spaced. For the usual situation of flow in a pipe or tube the electrodes are commonly spaced out across a diameter of the pipe and the magnetic field is provided by electro-magnets outside the pipe. Forces on the fluid that cause it to flow are increased if the magnetic field is increased or if the current through the electrodes is increased. In many applications of this technology the electro-magnetic pump is configured to offer a minimal additional impedance to the flow of fluid.
What is not found in the prior art is extensive teaching of electro-magnetic transducers for interacting with fluids flowing in tubes in which a trade-off is made between increased flow impedance and either higher flow meter output signals or increased pumping forces on the fluid. As an example of an application in which such a trade-off may be desirable, one may consider metering the flow of water in cases in which only moderate flow rates are experienced and unobstructed flow passage is relatively unimportant, but low cost and low power consumption for stand alone battery operation may be very important
Generally speaking, it is an object of the invention to provide an electro-magnetic transducer for interaction with a fluid in a tube, where the transducer comprises an electrically insulating sheet scrolled about an axis parallel to or coincident with an axis of the tube. An outer portion of the scroll may be adjacent an inner wall of the tube, and an inner portion of the scroll member may be very close to a streamlined body that is preferably coaxial with the tube. In addition, a transducer of this sort comprises at least one electro-magnet for providing a flux transverse to the tube; at least one pair of electrodes attached to the scroll member and defining a spiral electrical path extending along the scrolled sheet from one of the two electrodes to the other, and at least one electric circuit external to the tube for providing electric power to the at least one electro-magnet.
One version of the electro-magnetic transducer of the invention may be used in flow metering applications. In one preferred embodiment a magnetic flux generated by two electromagnets having magnetic cores is redirected by magnetic pole pieces so as to be orthogonal to both the axis between the electrodes and to a fluid flow direction. As is known in the magnetic flow metering art, the flux and the moving fluid will generate a voltage difference proportional to the flow rate of the fluid, the magnitude of the flux and the length of the conductive path between the electrodes. The voltage difference in the fluid is sensed by at least one, and preferably two pairs of electrodes arranged so that one pair is associated with each location of the pole pieces. In this embodiment, one of the electromagnets is located in a streamlined housing centered within the flow passage and the other electromagnet is on the outside of the passage so as to confine the flow to a quasi-annular ring. The pole pieces from the two magnets are located a selected distance apart and are aligned to reinforce their radial flux through the annular flow passage at two locations along the flow axis, thereby forming a complete magnetic circuit This annular flow passage is organized with a thin electrically insulating sheet wound into a scroll with a generally spiral cross-section This arrangement markedly increases the distance between the electrodes in each pair over what can be provided by the use of either diametrically opposed electrodes in an unobstructed tube, or by an arrangement using the quasi-annular ring of flowing fluid without the scroll.
In the flow meter described above a pair of electrodes is associated with each of the paired poles to sense the corresponding flow generated voltages. That is, flow signals are generated along a spiral path defined by the insulating scroll that is emplaced within the tube. In preferred embodiments, one pair of electrodes is located far enough from the other pair so that their signals have low mutual interaction. This arrangement can provide a combination of an increased fluid flow velocity, a longer path between electrodes and a highly efficient magnetic circuit These features enable a magnetic flow sensor to be produced having substantially greater flow-generated signals than is found in the prior art.
In considering the operation of the flow meter described above, one will recognize that the presence of the scroll and streamlined body within a flow passage reduce the cross sectional area of the flow passage and thereby increases the fluid flow rate at the expense of an increased pressure drop. At a fixed magnetic flux in the passage, the increased flow rate produces correspondingly higher electrode voltages than would be measured if the scroll and body were not there. Moreover, use of magnetic cores with pole pieces to provide a complete, shielded magnetic circuit concentrates the magnetic flux in the desired area. This arrangement enables a higher magnetic circuit efficiency to be achieved than is the case with commonly used air core magnets. Additionally, the magnetic field is generally confined to the annular flow passage in order to reduce problems of magnetic and electro-magnetic compatibility. The use of magnetic cores and pole pieces with prior art magnetic flow meters is generally not practical in larger pipe sizes because the orientation of the field would require a relatively large mass of core material that would increase the size and weight of the meters considerably.
In a preferred flow sensing embodiment, each electrode pair may be used with its own signal amplifying and processing circuitry to provide a flow rate signal. Alternately, signals from multiple pairs may be combined in various ways to provide redundancy and improved measurement precision. Each electrode pair may also be stabilized by short-circuiting the two electrodes of the pair together, or otherwise connecting both of the two electrodes to a common potential during the period when the magnetic field is not present, thereby further helping to reduce measurement errors.
In yet another preferred flow sensing embodiment, a reduction in overall power consumption for situations involving intermittent fluid flow is realized through the use of controller responsive to a second flow sensing means to detect an onset of flow and to then provide electrical power to the flow meter only during the periods of fluid flow.
A preferred pump embodiment of the transducer of the invention has a configuration similar to some of the flow sensing embodiments and employs the extended path provided by the use of a scrolled insulating body inserted into the tube so that the tube axis and the scroll axis are substantially coincident In this embodiment, an AC source supplies currents at the same frequency through both the electromagnets and the electrodes. When these currents are in phase, the fluid is pumped in one direction. When the currents are 180 degrees out of phase, the fluid is pumped in the opposite direction. Changing the currents through either the electromagnets or electrodes controls the magnitude of the pumping action.
Although it is believed that the foregoing recital of features and advantages may be of use to one who is skilled in the art and wishes to learn how to practice the invention, it will be recognized that the foregoing is not intended to list all of the features and advantages. Moreover, it may be noted that various embodiments of the invention may provide various combinations of the hereinbefore recited features and advantages of the invention, and that less than all of the recited features and advantages of the invention may be provided by some embodiments.